Add 2014 DMG encounter difficulty calculation

Support the 2014 DMG encounter difficulty as an alternative to the 5.5e
system behind the existing Rules Edition toggle. The 2014 system uses
Easy/Medium/Hard/Deadly thresholds, an encounter multiplier based on
monster count, and party size adjustment (×0.5–×5 range).

- Extract RulesEdition to its own domain module
- Refactor DifficultyTier to abstract numeric values (0–3)
- Restructure DifficultyResult with thresholds array
- Add 2014 XP thresholds table and encounter multiplier logic
- Wire edition from context into difficulty hooks
- Edition-aware labels in indicator and breakdown panel
- Show multiplier, adjusted XP, and party size note for 2014
- Rename settings label from "Conditions" to "Rules Edition"
- Update spec 008 with issue #23 requirements

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/__tests__/use-difficulty.test.tsx

@@ -12,6 +12,7 @@ import {
 } from "../../__tests__/factories/index.js";
 import { AllProviders } from "../../__tests__/test-providers.js";
 import { useDifficulty } from "../use-difficulty.js";
+import { useRulesEdition } from "../use-rules-edition.js";
 
 beforeAll(() => {
 	Object.defineProperty(globalThis, "matchMedia", {
@@ -81,7 +82,7 @@ describe("useDifficulty", () => {
 
 		await waitFor(() => {
 			expect(result.current).not.toBeNull();
-			expect(result.current?.tier).toBe("low");
+			expect(result.current?.tier).toBe(1);
 			expect(result.current?.totalMonsterXp).toBe(50);
 		});
 	});
@@ -223,9 +224,9 @@ describe("useDifficulty", () => {
 			expect(result.current).not.toBeNull();
 			// 1 level-1 PC: budget low=50, mod=75, high=100
 			// CR 1/4 = 50 XP -> low (50 >= 50)
-			expect(result.current?.tier).toBe("low");
+			expect(result.current?.tier).toBe(1);
 			expect(result.current?.totalMonsterXp).toBe(50);
-			expect(result.current?.partyBudget.low).toBe(50);
+			expect(result.current?.thresholds[0].value).toBe(50);
 		});
 	});
 
@@ -261,7 +262,7 @@ describe("useDifficulty", () => {
 		await waitFor(() => {
 			expect(result.current).not.toBeNull();
 			// 2x level 1: budget low=100
-			expect(result.current?.partyBudget.low).toBe(100);
+			expect(result.current?.thresholds[0].value).toBe(100);
 		});
 	});
 
@@ -304,7 +305,7 @@ describe("useDifficulty", () => {
 			expect(result.current).not.toBeNull();
 			// Thug CR 1 = 200 XP, Allied Guard CR 1 = 200 XP subtracted, net = 0
 			expect(result.current?.totalMonsterXp).toBe(0);
-			expect(result.current?.tier).toBe("trivial");
+			expect(result.current?.tier).toBe(0);
 		});
 	});
 
@@ -336,12 +337,57 @@ describe("useDifficulty", () => {
 			expect(result.current).not.toBeNull();
 			// Level 3 budget: low=150, mod=225, high=400
 			// CR 1/4 = 50 XP -> trivial
-			expect(result.current?.partyBudget.low).toBe(150);
+			expect(result.current?.thresholds[0].value).toBe(150);
 			expect(result.current?.totalMonsterXp).toBe(50);
-			expect(result.current?.tier).toBe("trivial");
+			expect(result.current?.tier).toBe(0);
 		});
 	});
 
+	it("returns 2014 difficulty when edition is 5e", async () => {
+		const wrapper = makeWrapper({
+			encounter: buildEncounter({
+				combatants: [
+					buildCombatant({
+						id: combatantId("c1"),
+						name: "Hero",
+						playerCharacterId: pcId1,
+					}),
+					buildCombatant({
+						id: combatantId("c2"),
+						name: "Goblin",
+						creatureId: crId1,
+					}),
+				],
+			}),
+			playerCharacters: [
+				{ id: pcId1, name: "Hero", ac: 15, maxHp: 30, level: 1 },
+			],
+			creatures: new Map([[crId1, goblinCreature]]),
+		});
+
+		// Set edition via the hook's external store
+		const { result: editionResult } = renderHook(() => useRulesEdition(), {
+			wrapper,
+		});
+		editionResult.current.setEdition("5e");
+
+		try {
+			const { result } = renderHook(() => useDifficulty(), { wrapper });
+
+			await waitFor(() => {
+				expect(result.current).not.toBeNull();
+				// 2014: 4 thresholds with Easy/Medium/Hard/Deadly labels
+				expect(result.current?.thresholds).toHaveLength(4);
+				expect(result.current?.thresholds[0].label).toBe("Easy");
+				// CR 1/4 = 50 XP, 1 PC (<3) shifts x1 → x1.5, adjusted = 75
+				expect(result.current?.encounterMultiplier).toBe(1.5);
+				expect(result.current?.adjustedXp).toBe(75);
+			});
+		} finally {
+			editionResult.current.setEdition("5.5e");
+		}
+	});
+
 	it("custom combatant with CR on party side subtracts XP", async () => {
 		const wrapper = makeWrapper({
 			encounter: buildEncounter({